Evolution of Interannual Warm and Cold Events in the Southeast Atlantic Ocean

Abstract

Extreme warm episodes in the southeast Atlantic Ocean, known as Benguela Nin os, have devastating environmental impacts and have been shown to be remotely forced. To place these extreme events into perspective, the investigation is here extended to minor warm events as well as to cold episodes. To this end, different sets of observations have been combined with outputs from a numerical simulation of the tropical Atlantic for the period 1982-99. It is shown that both warm and cold surface events develop regularly in the same specific region along the coast of Angola and Namibia. Some cold events compete in magnitude with major warm episodes. Local sea-air heat flux exchanges do not seem to precondition the sea surface in the Angola-Benguela region prior to the arrival of an event. Most warm and cold episodes are large-scale events despite their limited surface signature. They appear to be generated by wind anomalies in the western and central equatorial Atlantic in the same way as Benguela Nin os. Seasonal fluctuations of the depth and shape of the tropical thermocline seem partly to control the way subsurface anomalies eventually impact the surface. During austral summer, surface anomalies create an identifiable pool centered near 15S, whereas in winter they show an elongated pattern along the coast stretching toward the equator. Local upwelling or downwelling favorable wind regimes, as well as local net heat fluxes, may modulate the surface expression of events.